Polypropylene-based graft copolymers are useful as compatibilizers for a variety of polymer blends containing polypropylene. Polypropylene-based graft copolymers can be used as a blend component as well as an adhesion promoter between polyolefins and other substrates, including glass, metal, mineral fillers, polar polymers, and engineering plastics such as polyamides.
Functionalized polypropylene-based polymers can be produced by peroxide grafting of polypropylene backbones. During peroxide grafting of a polyolefin backbone, free radicals are produced. Such radicals not only trigger a grafting reaction onto a polyolefin backbone, but can also cause beta-scission of the backbone itself. The resulting molecular weight reduction becomes more severe as the degree of grafting and severity of the process conditions increases. The beta-scission reaction is especially prevalent in the neighborhood of tertiary carbon atoms in the polyolefin backbone chain. The production of highly functionalized propylene based backbones by peroxide grafting involves an appreciable loss of molecular weight, viscosity, and melt strength.
For many years, polypropylene (PP) has also been functionalized with maleic anhydride in presence of peroxide to produce maleic anhydride grafted polypropylene, which is used as an adhesion promoter in glass and mineral filled polypropylene compounds as well as compatibilizer of polyamide polypropylene blends. The grafted polypropylene-based polymers are also used in other applications where adhesion onto metal or polar substrates (including polar polymers) is required. Lately, grafted polypropylene has also found applications as coupling agents in natural fibers filled PP compounds. During the grafting process, macroradicals are generated and beta scission usually occurs before the reaction with maleic anhydride takes place. The result is that grafting levels are generally low and the resulting functionalized polypropylene has a low molecular weight. In order to obtain highly functionalized polypropylene-based polymers, it is necessary to increase the amount of peroxide which leads to further MW reduction. It has also been recognized in prior literature (M. Lambla et al. in Makromol. Chem., Macromol. Symp., 75, 137 (1993)) that the grafting yield of maleic anhydride is not a monotonic function of its initial concentration but reaches a maximum before decreasing. The existence of the maximum is associated with a limited solubility of maleic anhydride in the molten polypropylene. It is believed that with increasing the maleic anhydride feed, the polypropylene/maleic anhydride/peroxide mixture changes from a semi-homogeneous to a more heterogeneous system with maleic anhydride/peroxide droplets dispersed in the molten polypropylene.
EP 777 693 discloses a maleated polypropylene having an acid number greater than 4.5, a yellowness index color of no greater than 76, and a number average molecular weight of at least 20,000. The acid number can be translated into a wt % content of maleic anhydride. The number average molecular weight can be converted in co-dependence with the Mw/Mn ratio into weight average Mw which changes inversely to the MFR. While EP 777 693 aims to provide a relatively high molecular weight and a high degree of grafting without undue yellowing at the same time, the flexibility remains insufficient and significant molecular weight breakdown still takes place.
U.S. Pat. No. 5,670,595 relates to diene modified polymers to improve the melt strength of polypropylenes, low draw-down ratios in extrusion coating, poor bubble formation in extrusion foam materials, and relative weakness in large-part blow molding. The dienes are acyclic alpha-omega dienes. The starting polymer contains less than 5 mol % of other unsaturated compounds such as ethylene, butene-1 etc. customarily used for Random Propylene Copolymers (RCP) used generally as a heat seal layer on oriented polypropylene (OPP) film. Use of the invention described is alleged to limit the molecular weight reduction to less than 20% when the graft ratio is 0.7 wt %. Contacting in solution and in the molten condition are illustrated. The materials lack the flexibility and low glass transition temperature desirable to preserve good adhesion at low temperature and when deformed by flexing or impact.
The grafting of a broad range of olefin based polymers is discussed in U.S. Pat. No. 5,367,022. A high degree of grafting is suggested combined with low MFR (i.e., high molecular weight) polymer backbones. The examples show that the grafting still results in a polymer with an MFR well in excess of 100, which has inadequate melt strength and is unsuited for use in film extrusion if used as the predominant component of a composition. The homopolymers are crystalline, have an elevated heat of fusion before grafting, and possess limited flexibility.
U.S. Pat. No. 5,059,658 discloses a method of producing modified polypropylene having a Mw from 50000 to 1000000 and a graft ratio of 0.1 to 10 wt % by graft-polymerizing a substantially crystalline propylene random copolymer consisting essentially of propylene and a linear diene. Although, it is mentioned that the backbone can contain up to 5 mole % comonomer, there is no discussion of the level of crystallinity or isotacticity of the polymer to be grafted.
U.S. Pat. No. 5,763,088 reports olefin resin-based articles having gas barrier properties consisting of a maleic anhydride grafted polypropylene. The starting backbone can include a propylene copolymer with a C2-C8 alpha-olefin have a melting point between 80° C. and 187° C. and a degree of crystallinity of 20% or more. The object of this invention has a crystallinity level and melting points outside these ranges.
WO 2002/36651 describes the grafting of propylene based elastomers containing ethylene derived units to lower crystallinity. WO 2005/049670 discloses incorporating dienes into propylene-based elastomers but the grafting of such material themselves is not disclosed.
Apart from changes in the polymer backbone to be grafted and the grafting process, it has also been proposed to counteract any reduction in the molecular weight as a result of peroxide grafting by blending the propylene based polymer with a polyethylene which has a countervailing tendency of increasing its molecular weight as the result of a peroxide grafting process. If large amounts of polyethylene are used melt processability and compatibility with polypropylene substrates can be negatively affected. Similarly higher molecular weight ungrafted propylene and or ethylene based polymers can be added to a grafted polymer with a degraded molecular weight to restore the overall melt strength to a sufficient level. In practice, thus far, grafted propylene based polymer compositions for applications such as CTR have been made, in spite of the absence of high molecular weight grafted propylene based polymer materials, by blending low viscosity functionalized propylene based polymers with high molecular weight un-functionalized propylene based polymers, or by the use of electron donating agents during grafting such as DMF or styrene to reduce chain scissioning. See Gaylord, N. G., Mishra, M. K., J. Polym. Sci. B21, 23 (1983) and (styrene use): Hu, G. H. Flat, J-J, Lambla, M, Makromol. Chem., Macromol. Symp. 75, 137 (1993).
The effectiveness of the former compositions is however reduced by reduction of the grafting level and broadening of the molecular weight distribution. The use of these latter chemicals generates safety issues on typical reactive extrusion processes in their handling and feeding to the reaction device. They also require more extensive venting in order to minimize their residual level in the final functionalized polymer. These residuals can also be seen as contaminations which prevent the final polymer to be used in certain applications such as those requiring food contact classification.
There is a need, therefore, for a grafted polymer which combines a high content of propylene derived units for improved compatibility with propylene based materials as well as a high degree of grafting to improve adhesion. There is also a need for a grafted propylene-based polymer with sufficient flexibility to maintain adhesion under local deformation at the same time as a sufficiently high viscosity to give a melt strength needed for extrusion.